This application claims the benefit of Japanese Application No. 11-174239, filed in Japan on Jun. 21, 1999, which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a silica glass member, and more particularly, to a photolithography-use silica glass member used in an optical system, such as a lens or mirror, in photolithography in the wavelength band of less than about 400 nm, preferably less than about 300 nm.
2. Discussion of the Related Art
In recent years, higher integration and multi-functionality of VLSI (Very Large Scale Integration) have been significantly advanced. For example, in the field of logic VLSI, system-on-chip technology, in which a large system is incorporated in a chip, has been developed. Such a technological advance increasingly requires finer pattern formation and higher integration techniques on a wafer, such as a silicon wafer, which is a substrate for such chips. An exposure apparatus called xe2x80x9cstepperxe2x80x9d has been used in photolithography technique, which exposes and transfers a fine pattern of an integrated circuit onto a wafer (e.g., silicon wafer).
For example, for a DRAM, which is an example of VLSI, as the technology advances from LSI to VLSI, its capacity increases as 1Mxe2x86x924Mxe2x86x9216Mxe2x86x9264Mxe2x86x92256Mxe2x86x921G. Such an increase in capacity requires a photolithography apparatus that is capable of fine pattern fabrication as 1 xcexcmxe2x86x920.8 xcexcmxe2x86x920.5 xcexcmxe2x86x920.35 xcexcmxe2x86x920.25 xcexcmxe2x86x920.18 xcexcm, respectively, in terms of the fabrication line width.
Accordingly, projection lenses in the stepper are required to have a high resolving power and a deep focal depth. The resolving power and focal depth are determined by the wavelength of light used in exposure and N.A. (Numerical Aperture) of the lens.
The finer the pattern, the larger the angle of diffraction light. Thus, for a finer pattern, diffraction light cannot be properly included unless the lens has a sufficiently large N.A. On the other hand, the shorter the exposure wavelength xcex, the smaller the angle of diffraction light for the same pattern. Thus, the N.A. of a relatively smaller value may be sufficient.
The resolving power and the focal depth are expressed by the following formulae:
Resolving Power=K1xc2x7xcex/N.A.
Focal Depth=K2xc2x7xcex/N.A.2
where K1 and K2 are proportional constants.
Thus, to improve the resolving power, either N.A. can be increased, or xcex can be shortened. As seen from the above formulae, shortening of xcex is advantageous in terms of the focal depth. In light of this consideration, the wavelength of the light source has been shortened from the g-line (436 nm) to the i-line (365 nm), and further to KrF excimer laser (248 nm) and to ArF excimer laser (193 nm).
Silica glass, particularly synthetic silica glass manufactured by an oxygen/hydrogen flame hydrolysis method using SiCl4 as the material, has an extremely low metal impurities and a high transmittance with respect to ultraviolet light.
It has been required that silica glass used in an optical system for precision apparatus, such as projection lenses for photolithography, not have any striae in any direction. Accordingly, processes for removing striae have been necessary or special techniques have been introduced during the synthesis. Moreover, a cumbersome step of selecting desired products among silica glass produced has been required, which has caused a poor yield. Because of these extra steps, such silica glass has been very expensive as compared with silica glass of the normal optical grade, and particularly with a recent trend towards a larger diameter of projection lenses, the high cost associated with manufacture of projection lenses has posed a serious drawback in the industry.
The striae are displayed on a screen when silica glass is observed by a projection device using a pin-hole method, as shown in FIG. 1. In particular, a silica glass manufactured by the oxygen/hydrogen flame hydrolysis method tends to include striae in the form of stripes. This is due to the manufacture method, which involves depositing of SiO2 powder on a target or vessel, fusing, and vitrifying.
The method for measuring striae illustrated in FIG. 1 is similar to the striae measurement method specified in JOGIS 11-1975 in Japanese Optical Glass Industrial Standards, Appendix: Explanations on Measuring Method of Optical Glass, translated and published by Japan Optical Glass Manufacturers Association, which is hereby incorporated by reference. The striae measurement is conducted using the pin-hole method, the optical elements of which are arranged as shown in FIG. 1, and an interferometer technique, so-called xe2x80x9coil-on-plate methodxe2x80x9d Light emitted from a light source 1 passes through a slit 2 and a sample 3, and produces a striae image on a screen 4 in the form of stripes. The distance between the light source 1 and the pin-hole slit 2 is set to 4 m. As shown in FIG. 1, the strength of striae corresponds to the difference between the darkest part and the brightest part of the striae observed on the screen, and the spacing between striae is observed as the spacing between two adjacent peaks of the bright lines or dark lines. A standard sample having known striae is used to calibrate this striae measurement. Then, a striae pattern of a sample to be measured is observed in a manner shown in FIG. 1. As shown in FIG. 1, the strength of striae in terms of refractive index differential is derived from the width of the observed striae pattern, and an actual spacing between striae in the sample is derived from the observed spacing between the two adjacent peaks in the observed striae with reference to the calibration sample.
Accordingly, the present invention is directed to a silica glass member that substantially obviates the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide a silica glass member with a lower cost without sacrificing its focusing characteristics.
Additional features and advantages of the invention will be set forth in the description that follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, the present invention provides a silica glass member for use in an optical system using light of a wavelength equal to or less than about 400 nm as a light source, the silica glass member having striae in a direction different from an optical axis of the optical system, the strength of the striae being equal to or less than about 2xc3x9710xe2x88x926 in terms of refractive index differential.
In another aspect, the present invention provides an exposure apparatus having an illumination optical system for directing light from a light source towards a mask having a pattern thereon and a projection optical system for projecting an image of the pattern on the mask towards a substrate to expose the image on the substrate, wherein the silica glass member of the present invention is installed in at least one of the illumination optical system and the projection optical system.
In a further aspect, the present invention provides an optical system having an optical axis, including a silica glass member for processing light of a predetermined wavelength along the optical axis, the silica glass member having striae extending in a direction substantially perpendicular to the predetermined optical axis of the optical system, the strength of the striae being smaller than about 2xc3x9710xe2x88x926xc3x97(the value of the predetermined wavelength of the light in nanometers)/632.8 (nm) in terms of refractive index differential.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory, and are intended to provide further explanation of the invention as claimed.